This invention relates to thermal transfer printing, and more particularly to an ink composition for thermal transfer printing in which a colorant is uniformly dispersed.
Thermal transfer printing is used extensively in facsimiles, computer terminals, recorders and printers since printing is non-impact, operation is noiseless and maintenance free, cost is low, design is compact and light weight, and printing may be colored. Particularly, thermal transfer printing using an electric head is very suitable for color printing with half-tones and is receiving increased attention.
A suitable ink for thermal transfer printing must be of the hot-melt type in order to insure rapid phase change in the ink during printing (solid phase-molten phase-solid phase). Accordingly, it is necessary to disperse a colorant such as a dye or pigment in a natural or synthetic wax composed mainly of hydrocarbons and then add a defined amount of synthetic resins and plasticizers to improve adhesion and film strength.
However, it is difficult to disperse a colorant uniformly in a medium composed mainly of wax. Although mixers such as butterfly mixers, planetary mixers, sandmill mixers, three-roll mixers and attritors have been used, they are not fully satisfactory. Even if a high-shear force is attained during mixing, colorant is not uniformly dispersed due to wax in the medium.
If colorant is not uniformly dispersed in the medium, the resulting ink composition has many disadvantages. Ink density is not uniform which reduces print quality, particularly in full-color printing. Ink transparency is also reduced, resulting in lower croma in full-color printing. Furthermore, ink cohesion is lowered, thereby reducing ink transfer to the recording paper during printing. This problem also reduces print quality since density is not uniform and print surface gloss is reduced on the recording paper.
Accordingly, it is desirable to provide an improved ink for thermal transfer printing in which colorant is uniformly dispersed.